As Iraq enters a new phase of civil struggle, it is worth remembering how the nation arrived at its present desperate condition. The clamor for the United States to do something, or do nothing, or to find someone to blame for everything that has gone wrong, is likely to grow louder as the death toll mounts in Iraq. Clarity will be important.

It is important to remember, first of all, that Iraq’s new era began with the invasion launched in 2003 by President George W. Bush. The lies used to justify the war and the crimes committed in carrying it out are well known and represent a nightmare that the nation had lately thought it might be able to wake from. But the nightmare continues.

Saddam Hussein was a vicious dictator — there is no disagreement there. He had come to power with the Baath Party, which was dominated by Sunni Muslims. He slaughtered Iraqi Kurds and Shiite Muslims, and initiated a war with Iran that cost hundreds of thousands of lives. Because we viewed Iran as our enemy, the Reagan administration became friendly with Saddam. Then Saddam invaded Kuwait, causing President George H.W. Bush to launch the first Gulf War to expel him.

The U.S. invasion of Iraq allowed for the removal of Saddam and the banishment of the Baath Party. It also destroyed the authority and power of the Iraqi government, unleashing sectarian conflict that Saddam’s iron fist had repressed. Civil war between Sunni and Shia followed, with tens of thousands of deaths and millions of refugees.

The surge of U.S. troops under President George W. Bush helped create conditions for the Sunni Awakening. Sunni tribesmen who were at war with the Shia majority were paid by the United States to turn their hostility toward Islamic extremists in their midst and to allow for the creation of an Iraqi government under the tutelage of the United States. Eventually, Nouri al-Maliki became prime minister with the idea that Shia, Sunni and Kurds would all participate in the new government.

But Maliki has ruled as a Shia henchman, excluding and persecuting Sunnis, and the Sunnis have been joined by Islamic extremists and former backers of Saddam to seize major cities. The war between Sunnis and Shias that claimed many thousands of lives in the mid-2000s has been rekindled, and the same horror now wracking Syria may be in store for Iraq.

There is zero chance that the American people are ready to revive the nightmare of deception and thuggery perpetrated by the Bush administration. And yet a lingering trace of responsibility still attaches to the United States because of our role in setting the whole disaster into motion. Walking away from that responsibility would brand us as hopelessly feckless. But what can we do?

The dangerous faction spearheading the latest rebel advances is a Taliban-like movement given to extreme brutality and repression. Somehow the United States should use its power to enlist the old coalition of Sunnis, Shias and Kurds in blunting the advance of the extremists. To do so, President Barack Obama must be able to convince Maliki or those around him that their divisive approach to governing has brought down on their heads the very disaster they were trying to avert. U.S. arms and wealth are weapons. Some sort of true coalition between Sunni and Shia is a must. Apparently, the Kurds are happy edging closer to true independence for their own region.

Obama’s instincts for avoiding disaster are sound, but they leave him open to charges that he is weak. Being smart, however, makes us stronger and helps grant to us the initiative. That strength ought to be used, not to wall off the United States from the chaos of Iraq but to help where help is possible and to guide leaders in a direction that will avoid the worst forms of disaster. It will count as success if Obama can help Iraq avoid the fate of Syria, which has been crushed by civil war. Rushing in again with a heavy American presence would be to repeat costly old mistakes.